1. Field of the Invention
The present invention relates to a method and apparatus for constraining a shaft to rotate about an axis when held between two bearings. It further relates to such a method and apparatus where it is required to control the rotation-opposing friction despite variations imposed in bearing load due to thermal and other dimensional changes in the equipment. The invention is particularly useful for mounting rotary head positoning actuators in Winchester style disc drives for their rotation about the axis of a shaft to position one or more data recording or data recovering heads at selectable radii on rotating data storage discs.
While the invention is hereinafter described in relation to such an application within a Winchester disc drive, such application is not intended as a limitation to its utility.
2. The Prior Art
A winchester style disc drive comprises a base whereon one or more discs are restrained to rotate on a spindle and whereon a rotary actuator is mounted, the actuator comprising a head mounting arm rotatable about an axis to position a head or heads for data storage or recovery at selectable radii on the disc or discs.
The rotary actuator in general comprises a yoke having bearings mounted into the opposite faces thereof for the rotation of a shaft passing therethrough, the head arm being keyed to or integral with the shaft. It is general that the shaft is an interference fit in the bearing and the bearing is an interference fit within its particular face of the yoke.
During assembly of the yoke and shaft with the bearings, because of the frictional, transition fit of the various parts it is difficult to control the residual force directed along the axis of the shaft as the bearing load. Similarly, when the assembly is subjected to change in temperature, the relative expansion and contraction between the shaft and the yoke can set up very large bearing loads. As bearing load increase, so the amount of torque required to turn the shaft about its axis becomes greater. In the case of the rotary actuator in a Winchester disc drive, the increasing torque is reflected in an increasing positioning error for the heads as the friction encountered by the head positioning servomechanism of which the actuator forms part rises. When the friction exceeds a critical limit the head position error induces data corruption so that recorded data cannot be recovered or new data is written unwantedly and destructively in the position of data it was desired to retain. The difficulty in controlling residual bearing loads makes it difficult to predict when an assembly is liable to exceed the critical limit and it is perfectly and distressingly possible to manufacture an assembly which passes all tests only to fail after delivery to a customer.
The bearings for the actuator shaft must be precise. There are many hundreds of data storage tracks per radial centimeter on the disc of a Winchester drive, and the accuracy of positioning must be such that the head or heads can be positioned to intercept a selectable one of them. The amount of play in the bearings is correspondingly small and preferably zero, making the takeup of stress or thermally induced dimensional changes by residual play an impractical proposition. It is not possible to deliberately make the transition fits of poor frictional quality, since this would allow the shaft to slip during shipping or working life of the drive and render the actuator inoperable. As an added disadvantage, the precision of the dimensions required to provide a transition fit of controlled friction is such as to render cost prohibitive.
It is therefore desirable to provide a method and apparatus for the mounting of a shaft between two bearings in a yoke where the shaft is free to rotate about an axis with less than a predetermined amount of frictional torque despite thermal expansion and assembly tolerances and with substantially no play in a transverse or longitudinal direction relatively to said axis.